Multiphase morphospace saturation in cyrtocrinid crinoids

Abstract

The study of the relationship between disparity (occupied morphospace) and diversity (number of taxa) through geological time represents a powerful tool in the macroevolutionary study of groups. In this contribution, this approach is applied for the first time to the cyrtocrinid crinoids, a major clade of mostly Mesozoic articulate crinoids also represented by rare Cenozoic forms (two extant taxa). The analysis of disparity identified two separate evolutionary radiations for cyrtocrinids with maximum morphospace exploration, one at the beginning of the evolutionary history of the group in the Pliensbachian and a second one between the Late Jurassic and Early Cretaceous. On the methodological level, the disparity measured both as total variance and as sum of ranges shows compatible results, with trends well coupled to the diversity curve indicating that, in cyrtocrinid crinoids, an increase or decrease in the number of taxa in the history of the clade corresponds a proportional increase and decrease also in the occupied morphospace. The curves obtained were interpreted in the light of the clade's phylogeny, major oceanographic events, newly available ecological niches and relative key innovations, which would be able to increase the fitness of the group. The group diversity was already in decline starting from the Aptian, and the mass extinction at the K‐PG boundary had no effect on the history of the clade. The results show once again the importance and potential of diversity/disparity studies when put into the light of palaeotectonic, palaeoecological and palaeoenvironmental factors.

First remains of Lepisosteiformes actinopterygians from the Jurassic of Monte Nerone area (Umbria-Marche Apennine, Italy)

“…As already stressed for the evolution of cyrtocrinid crinoids (Manni & nicoSia, 1996;roMano et alii, 2016;2018b) evolutionary radiations could be linked to large scale geodynamic phenomena in the UMS Domain, as the formation of submarine fault-scarps that connected the hanging-walls with the tops of structural highs. As reported in roMano et alii (2018a), the complexity of the Jurassic submarine palaeotopography with deeper basins and PCPs sensu Santantonio (1994) could led to the exploration of new infaunal and epifaunal communities and, thus, also to new unexplored trophic niches for durophagous organisms.…”

Morphospace saturation in the stem-gnathostomes pteraspidiformes heterostracans: an early radiation of a ‘bottom’ heavy clade

“…Important in this context is whether timing of morphospace occupations as either early or late in the history of a clade and how they compare to changes in taxic diversity. Studies of morphospace occupation in both invertebrates (e.g., Foote, 1994 ; Foote, 1999 ; Lofgren, Plotnick & Wagner, 2003 ; Villier & Eble, 2004 ; Lefebvre et al, 2006 ; Al-Sabouni, Kucera & Schmidt, 2007 ; Scholz & Hartman, 2007 ; Glaubrecht, Brinkmann & Pöppe, 2009 ; Whiteside & Ward, 2011 ; Deline & Ausich, 2011 ; Bapst et al, 2012 ; Hopkins, 2013 ; Romano et al, 2018 ) and vertebrates (e.g., Prentice, Ruta & Benton, 2011 ; Benson, Evans & Druckenmiller, 2012 ; Ruta et al, 2013 ; Colombo et al, 2015 ; Marx & Fordyce, 2015 ; Larson, Brown & Evans, 2016 ; Romano, 2017a ; Romano, Brocklehurst & Fröbisch, 2017 ) have reconstructed the timing of radiations, with many identifying maximum disparity at the beginning of their evolutionary history (termed ‘bottom heavy’), followed by stabilization and constant decrease until their subsequent extinction (e.g., Gould, Gilinsky & German, 1987 ; Foote, 1992 ; Foote, 1994 ; Foote, 1995 ; Foote, 1999 ; McGhee Jr, 1995 ; Wagner, 1995 ; Smith & Bunje, 1999 ; Eble, 2000 ; Huntley, Xiao & Kowalewski, 2006 ; Ruta et al, 2013 ; Marx & Fordyce, 2015 ; Romano, 2017a ). Here we test the timing of morphospace radiations for Pteraspiformes and compare that to taxic diversity.…”